In the papermaking art, an aqueous suspension containing cellulosic fibers, and optional fillers and additives, is fed into a headbox which ejects the cellulosic suspension onto a forming wire. Water is drained from the cellulosic suspension to provide a wet paper web which is further dewatered and dried in the drying section of the paper machine. Drainage and retention aids are conventionally introduced into the cellulosic suspension to facilitate drainage and increase adsorption of fine particles onto the cellulosic fibers so that they are retained with the fibers.
Sols of silica-based particles are widely used as drainage and retention aids, usually in combination with charged organic polymers. Such additive systems are among the most efficient now in use in the papermaking industry, in particular those comprising silica-based sols which contain microgel or aggregated particles of high surface areas. Examples of silica-based sols of this type include those disclosed in U.S. Pat. Nos. 5,176,891; 5,368,833; 5,603,805 and 6,372,806 as well as International Patent Appl'n Pub. Nos. WO 98/30753; 98/56715; 00/66491; 00/66492; 2005/097678 and 2005/100241.
Spherical silica-based particles can grow and aggregate in various ways depending on the conditions. Under certain conditions, the particles grow symmetrically, thus maintaining a spherical shape. Under other conditions, the spherical particles aggregate to clusters of particles and form three dimensional networks and microgels. Silica-based particles may also form elongated aggregates that are more or less linear, thus forming aggregates with different degrees of aggregation in different directions or axes.
High surface area aqueous silica-based sols containing microgel usually have poor stability and high dilution is normally necessary to avoid complete gelation. Because of the stability problems associated with such products, and the prohibitive cost of shipping stable but extremely dilute products, high surface area aqueous silica-based sols containing microgel are preferably prepared at the location of intended use, for example at the paper mill.
Sols of aggregated silica-based particles can be defined by means of different parameters, including S-value and axial ratio. The S-value indicates the degree of aggregate or microgel formation; a lower S-value is indicative of a higher degree of aggregation of the silica-based particles. The axial ratio is applicable to elongated aggregates of silica particles and indicates the ratio of the long axis to the short axis.
It would be advantageous to be able to provide silica-based sols with improved drainage and retention performance. It would also be advantageous to be able to provide silica-based sols and, in particular, aggregate or microgel containing silica-based sols with improved surface area stability at very high surface areas and SiO2 contents. It would also be advantageous to be able to provide a method for producing such silica-based sols. It would also be advantageous to be able to provide a papermaking process with improved drainage and retention performance.